[time-nuts] OCXO and 50Ω termination

Bob kb8tq kb8tq at n1k.org
Wed Sep 27 14:20:35 EDT 2017


A lot depends on how the output stage in the OCXO was designed. Unless
you know the details of “what’s inside” it’s best to terminate it in the specified 

One example: Tuned tank in the collector of the output stage driving a matching 
network. Terminate it properly and the stage is not clipping (either voltage
or current wise). Terminate it in an arbitrarily wrong way and the stage goes
into voltage clipping. The noise then degrades, the isolation degrades, That in 
turn impacts TC, and load stability. If you have a multiplier involved, the
sub-harmonics change (rarely for the better). 

Another example: +25 DBM output stage driving a 15 db pad to give you
10 DBM out. Terminate it any way you want, it isn’t going to care. 

No single “one size fits all” answer. The fancier the specs on the OCXO, 
the more likely they are to look like the first example. It’s tough to get 
-185 doc / Hz phase noise when you have a 15 db pad and 10 dbm out. 


> On Sep 27, 2017, at 1:52 PM, Attila Kinali <attila at kinali.ch> wrote:
> Moin,
> Most OCXOs have a 50Ω output, which suggests that they expect to be
> terminated with 50Ω. Now on a normal PCB the wire from the OCXO to
> the rest of the circuit is usually rather short (1-5cm) which means
> that it is much less than the wavelength of the 10MHz output. Even 
> when looking reflections, a 2*5cm path (ie forward and back again)
> would be less than 500ps (in the order of 300ps for FR4). My guess
> would be that a non-50Ω termination would not result in any adverse
> effects as the paths are short and the reflections would be constant.
> The changes that would affect the delay would be temperature and
> humidity (mostly humidity in this case) which are both rather slow
> and my guess would be that the added instability would be drowned
> in the temperature and aging drift of the OCXO.
> So, I'd say that it would be still ok to use 1-10kΩ termination
> impedance without any problems. Is this assumption correct?
> Is there anything I am missing?
> 				Attila Kinali
> -- 
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